(a) Field of the Invention
The invention relates to a high density rewritable phase-change optical storage media, and more particularly, to a phase-change optical storage media which exhibits a reduced degradation during repeated overwriting.
(b) Description of the Related Art
Recently, an increase in the amount of information data demands a high-density storage media which permits a recording/playback of a vast amount of information rapidly. It is expected that optical storage media would meet the need of such applications.
An optical disc includes a write-once type which allows a recording operation only once and a rewritable type which allows an overwriting as many times as desired. A rewritable optical disc includes a magneto-optical disc which utilizes the magneto-optical effect and a phase-change disc which utilizes a change in the reflectivity associated with a reversible phase-transformation between crystallized and amorphous states. A phase-change disc does not require an external magnetic field and enables a recording/erasure by merely modulating the power of a laser irradiation, thus presenting an advantage that a recording/playback unit can be constructed in a compact size. It also affords an advantageous possibility that a higher density can be achieved by using an irradiation source of shorter wavelengths, without modifying materials used in a recording layer or the like in a conventional disc recorded and erased with a currently dominant wavelength on the order of 800 nm,.
A material for a recording layer of phase-change type often comprises a thin film of chalcogen alloy such as GeSbTe, InSbTe, GeSnTe, AgInSbTe etc., for example. In a rewritable recording disc of phase change type which is currently implemented for practical use, a unrecorded (or erased) state is represented by a crystallized state, whereas recorded state is represented by an amorphous state. The amorphous bit is formed by heating the recording layer to a temperature higher than the melting point, followed by quenching. To prevent an evaporation and/or deformation from occurring as a result of such heat treatment of the recording layer, the recording layer is usually sandwiched by heat-resistant and chemically stable dielectric protective layers which are disposed on the opposite sides thereof.
During a recording process, the protective layers promote a thermal diffusion from the recording layer to achieve a suprer-cooled condition, thus contributing to the formation of an amorphous bit. A metallic reflective layer is generally provided on the sandwich structure to provide a quadri-layer structure, which further promotes the thermal diffusion to insure the amorphous mark formation. Erasure (or recrystallization) takes place by heating the recording layer to a temperature above the crystallization temperature, but below the melting point. In this instance, the dielectric protective layers act as heat accumulating layers.
For a so-called one-beam overwritable phase-change disc, both the erasure and re-recording process can be simultaneously achieved by the intensity modulation of a single focused light beam. (See J. Appl. Phys., 26(1987) Suppl. 26-4, pp.61-66.) With the one-beam overwritable phase-change disc, the layer construction of the recording disc and the circuit arrangement of the drive can be simplified, thus drawing attention for its use as an inexpensive high density and high capacity recording system.
The recording process for the phase-change disc involves an extreme thermal stress cycle that forcibly melts the recording layer and then quenches it below the melting point within several tens of nanoseconds. For this reason, even if the recording layer is sandwiched by the dielectric protective layers, a repeated overwriting operation as many as several thousands or several tens of thousands times builds up a microscopic deformation or segregation in the recording layer, eventually leading to an increase of optically recognizable noise and the formation of local defects of micron order size. (see J. Appl. Phys., 78(1995), pp.6980-6988.) While a substantial improvement is achieved through a sophistication in respects of the recording layer material, the material for the protective layers or layer structure, there is an essential upper limit on the number of overwriting operations, which is by one order of magnitude or more below the number of overwriting operations available with a normal magnetic recording disc, or magneto-optical recording disc.
The degradation which results form the repeated overwriting operations depends on the configuration of a groove. To give an example, a rewritable compact disc (CD-Rewritable or CD-RW) is recently proposed ("CD-ROM Professional" in the United States, September 1996, pp.29-44 or Assembly of Manuscripts for phase-Change Optical Recording Symposium, 1995, pp.41-45.).
For a CD, a train of pits formed in a substrate at a pitch of 1.6.+-.0.1 .mu.m is scanned from the back side of the substrate by a focused light beam having a wavelength of 780.+-.30 nm to retrieve information. It is prescribed that the reflectivity in a non-pit area be equal to or greater than 70%.
For CD-RW, although the compatibility with CD inclusive of as high a reflectivity as 70% or more is difficult to achieve, the compatibility with CD can be secured in respect of the recorded signal and groove signal as long as the requirement for the reflectivity is allowed to be above 15% and below 25% for a non-recorded area and below 10% for a recorded area. The compatibility can be secured within the reach of the current CD drive technology if an amplification system which compensates for a reduced reflectivity is added in a playback system.
In a CD-RW, the groove is used as a recording track, and a record is made within the groove. It is proposed that a wobble containing address information can be used in the groove (JP-A-1993-210,849). FIGS. 1A and 1B illustrate a schematic view of such a disc. Wobbled grooves 11 are spaced apart in the surface of substrate and are separated from each other by inter-groove (land) spaces 12. It is to be noted that the amplitude of the wobble is shown exaggerated. The wobble is formed by a frequency modulation using a carrier frequency of 22.05 kHz. The wobble amplitude is very small in comparison to the pitch of the groove 11, which is a distance measured between imaginary centerlines of grooves 11 located on the opposite side of the inter-groove space 12 and is normally on the order of 1.6 cm, and is on the order of 30 nm.
A frequency modulation of the wobble in accordance with absolute time information or address information is referred to as ATIP (Absolute Time In Pre-groove) or ADIP (Address In Pre-Groove), and is already utilized in a recordable compact disc (CD-Recordable or CD-R) and mini-disc. (See "CD Family" by Heitaro Nakajima, Takao Ihashi and Hiroshi Ogawa, Published from OHM-sha in 1996, Chapter 4, and Proceedings of the IEEE, Vol. 82(1994). Page 1490.)
It is found by an investigation by the present inventors that repeated overwriting operations produce a new degradation phenomenon that the deteriorated wobble signal leaks into the recorded signal. This further reduces the repeatable times, by one order of magnitude or more, down to the order of 1000. In a CD-RW, the wobble is used to provide an address information which is required in detecting an unrecorded region into which information is to be recorded . This phenomenon, which limits the number of times for the overwriting operation, will present serious problem when the wobble is used in a disc having a high track density.
On the other hand, it will be seen that what is most frequently recorded in repeated manner in a usual recording disc will be a rewriting of file management area which stores content information for user data, and it is unlikely that the user data itself is rewritten as many as 1000 times. By way of example, considering a CD format, it is likely that TOC (Table Of Contents) region or PMA (Program Memory Area) region in a rewritable CD disc is frequently rewritten. Such a file management area is a small limited region which is disposed along the innermost or outermost periphery of the entire recordable region of the optical disc, and remains to be less than several percents of the entire recordable region; a degradation attributable to the wobbled groove presents a problem mainly in the TOC region as far as the CD format is concerned. However, this represents a very important region in which the content of the user data is recorded. Once an error occurs in this region, there results a failure to read data from the entire data area, and the disc can no longer be used, thereby limiting the life of the disc. To cope with this problem, a reserve track may be secured for use as a substitute for the file management area so that the substitute track may be used when an increased number of errors occur as a result of overwriting operations. However, still there is a limit on the number of overwriting operations that can be used, and in addition, a procedure of the file management is troublesome, presenting a difficulty in the design for drives and device drivers. In other words, the actual circumstance is that the number of overwriting operations which can be repeated for the entire disc is limited due to the presence of a region which is frequently rewritten and which occupies less than several percents.